The polyphenylene ether resins and methods for their preparation are well known to the polymer art. These polymers may be made by a variety of catalytic and non-catalytic processes from the corresponding phenols or reactive derivatives thereof. By way of illustration, certain of the polyphenylene ethers and methods of preparation are disclosed in Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875, and in Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358. In the Hay patents, the polyphenylene ethers are prepared by an oxidative coupling reaction comprising passing an oxygen-containing gas through a reaction solution of a phenol and a metal-amine complex catalyst. Other disclosures relating to processes of preparing polyphenylene ether resins, including graft copolymers of polyphenylene ethers with styrene type compounds, are found in Fox, U.S. Pat. No. 3,356,761; Sumitomo, U.K. Pat. No. 1,291,609; Bussink et al., U.S. Pat. No. 3,337,499; Blanchard et al., U.S. Pat. No. 3,219,626; Laakso et al., U.S. Pat. No. 3,342,892; Borman, U.S. Pat. No. 3,344,166; Hori et al., U.S. Pat. No. 3,384,619, Faurote et al., U.S. Pat. No. 3,440,217; and disclosures relating to metal-based catalysts which do not include amines, are known from patents such as Wieden et al., U.S. Pat. No. 3,442,885 (copper-amidines); Nakashio et al., U.S. Pat. No. 3,573,257 (metal-alcoholate or -phenolate); Kobayashi et al., U.S. Pat. No. 3,455,880 (cobalt chelates); and the like. All of the above-mentioned disclosures are incorporated herein by reference.
Catalyst systems based on cupric or cuprous salts and primary, secondary, or tertiary amines, with additives such as alkali or alkaline earth metal bromides and quaternary ammonium salts, have been disclosed. See, for example, Cooper et al., U.S. Pat. No. 3,733,299 and Bennett et al., U.S. Pat. No. 3,977,297, both of which are incorporated by reference. When the amine portion of the catalyst is di-n-butylamine, cupric salts are more active than cuprous salts, especially when used in conjunction with alkali or alkaline earth metal bromides. Particularly active catalyst systems are those employing cupric bromide or cupric chloride with additional sodium bromide, preferably in methanol, as a vehicle for the catalyst, with di-n-butylamine.
There are, however, some difficulties encountered in the use of these copper salt-alkali metal bromide catalysts. One difficulty is the tendency of catalyst solutions in methanol when they are allowed to contact water, to lose activity, thus requiring larger amounts of catalyst to give acceptable molecular weight in the polyphenylene oxide. Another difficulty is that the anhydrous cupric salts are not readily available commercially and are generally expensive to obtain. A third difficulty is that when methanol is used as the vehicle for the catalyst, it is extracted with the catalyst, after reaction, by aqueous solutions of acids or chelating agents, thus requiring an expensive distillation operation to recover the methanol from these aqueous solutions for economic and environmental reasons.
It has been found that an effective catalyst for the oxidative coupling can be obtained without the use of methanol, from inexpensive components, by mixing a copper oxide, preferably cuprous oxide, with concentrated aqueous hydrobromic acid and a secondary monoamine. The resulting catalyst from this mixture gives improved catalyst activity over cuprous salts, even when these cuprous salts are used in conjunction with alkali or alkaline earth metal bromides. The activity is comparable to that obtained by the use of cupric salts with alkali or alkaline earth metal bromides in methanol.
The diamine catalyst system--a combination of a copper salt with two amines, a hindered secondary diamine (N,N'-di-tert-butylethylenediamine) and a tertiary amine with low steric requirements (n-butyldimethylamine)--is highly active, typically requiring only one-half to one-third the copper concentration of the usual combinations of copper salts with monoamines. It does, however, have the disadvantage of being temperature-sensitive. If the reaction mixture is allowed to remain for more than a few minutes at temperatures greater than about 85.degree. F., the catalyst becomes deactivated and the polymerization reaction stops. In large scale operation, limitations of heat exchanger capacity and efficiency make it very difficult to control the reaction temperature at this level during the initial, exotherm period of the reaction, during which the monomer is added to the reactor.
The temperature sensitivity of diamine catalyst systems can be decreased by adding methanol to the reaction mixture. However, when methanol is present, large amounts of water must be added to the reaction mixture in the step in which the catalyst is extracted prior to isolation of the polymer. The methanol must then be separated by distillation of the methanol-water phase.
It is desirable to be able to employ cupric salts as catalysts since these salts generally have better solubility in aqueous hydrobromic acid media, the form in which the copper is most effectively employed, and since a variety of inexpensive cupric salts are commercially available. It has been discovered that cupric salts can be effectively utilized as catalysts for the preparation of polyphenylene ethers with the diamine catalyst system in which no methanol is employed, at higher temperatures, by adding low levels of a promoter consisting of a sulfite or bisulfite salt to the catalyst premix consisting of the cupric salt and hydrobromic acid, prior to addition of this catalyst premix to the reactor.